Friday the 13th, 2029

May 13, 2005: Friday the 13th is supposed to be an unlucky
day, the sort of day you trip on your shoe laces or lose your wallet
or get bad news.

But maybe it's not so bad. Consider this: On April 13th--Friday
the 13th--2029, millions of people are going to go outside, look
up and marvel at their good luck. A point of light will be gliding
across the sky, faster than many satellites, brighter than most stars.

For
a while astronomers thought it might. On Christmas Eve 2004, Paul
Chodas, Steve Chesley and Don Yeomans at NASA's Near Earth Object
Program office calculated a 1-in-60 chance that 2004 MN4 would collide
with Earth. Impact date: April 13, 2029.

The asteroid is about 320 meters wide. "That's big enough to
punch through Earth's atmosphere," devastating a region the size
of, say, Texas, if it hit land, or causing widespread tsunamis if
it hit ocean, says Chodas. So much for holiday cheer.

Asteroid 2004 MN4 had been discovered in June 2004, lost, then discovered
again six months later. With such sparse tracking data it was difficult
to say, precisely, where the asteroid would go. A collision with Earth
was theoretically possible. "We weren't too worried," Chodas
says, "but the odds were disturbing."

This is typical, by the way, of newly-discovered asteroids. Step 1:
An asteroid is discovered. Step 2: Uncertain orbits are calculated from
spotty tracking data. Step 3: Possible Earth impacts are noted. Step
4: Astronomers watch the asteroid for a while, then realize that it's
going to miss our planet.

Astronomers knew 2004 MN4 would miss Earth when they found pictures
of the asteroid taken, unwittingly, in March 2004, three months before
its official discovery. The extra data ruled out a collision in 2029.

Instead, what we're going to have is an eye-popping close encounter:

On April 13, 2029, asteroid 2004 MN4 will fly past Earth only 18,600
miles (30,000 km) above the ground. For comparison, geosynchronous
satellites orbit at 22,300 miles (36,000 km). "At closest approach,
the asteroid will shine like a 3rd magnitude star, visible to the
unaided eye from Africa, Europe and Asia--even through city lights,"
says Jon Giorgini of JPL. This is rare. "Close approaches by
objects as large as 2004 MN4 are currently thought to occur at 1000-year
intervals, on average."

Above:
The trajectory (blue) of asteroid 2004 MN4 past Earth on April 13,
2029. Uncertainty in the asteroid's close-approach distance is represented
by the short white bar. [More]

The asteroid's trajectory will bend approximately 28 degrees during
the encounter, "a result of Earth's gravitational pull,"
explains Giorgini. What happens next is uncertain. Some newspapers
have stated that the asteroid might swing around and hit Earth after
all in 2035 or so, but Giorgini discounts that: "Our ability
to 'see' where 2004 MN4 will go (by extrapolating its orbit) is so
blurred out by the 2029 Earth encounter, it can't even be said for
certain what side of the sun 2004 MN4 will be on in 2035. Talk of
Earth encounters in 2035 is premature."

In January 2004, a team of astronomers led by Lance Benner of JPL
pinged 2004 MN4 using the giant Arecibo radar in Puerto Rico. (Coincidentally,
the Arecibo dish is about the same size as the asteroid.) Echoes revealed
the asteroid's precise distance and velocity, "allowing us to
calculate the details of the 2029 flyby," says Giorgini, who
was a member of the team along with Benner, Mike Nolan (NAIC) and
Steve Ostro (JPL).

More data are needed to forecast 2004 MN4's motion beyond 2029. "The
next good opportunities are in 2013 and 2021," Giorgini says.
The asteroid will be about 9 million miles (14 million km) from Earth,
invisible to the naked eye, but close enough for radar studies. "If
we get radar ranging in 2013, we should be able to predict the location
of 2004 MN4 out to at least 2070."

Right:
The Arecibo radar in Puerto Rico is coincidentally about the size
of asteroid 2004 MN4. [More]

The closest encounter of all, Friday the 13th, 2029, will be a spectacular
opportunity to explore this asteroid via radar. During this encounter,
says Giorgini, "radar could detect the distortion of 2004 MN4's
shape and spin as it passes through Earth's gravity field. How the
asteroid changes (or not) would provide information about its internal
structure and material composition." Beautifully-detailed surface
maps are possible, too.

The view through an optical telescope won't be so impressive. The
asteroid's maximum angular diameter is only 2 to 4 arcseconds, which
means it will be a starlike point of light in all but the very largest
telescopes.

But to the naked eye--wow! No one in recorded history has ever seen
an asteroid in space so bright.